There are a few places you find the API key(s) associated with your user account.
All API Keys
The best place to find all of your API keys is in your profile. To get to your profile, you can click My Profile in the drop-down menu available which clicking your user's profile image in the upper-right of each page.
Once you are on your profile, you can switch to the Credentials tab to see your default API key as well as have the ability to create new API keys.
Default API Key
There are several places around Algorithmia that will help you with displaying your default API key. For example, in the Getting Started walkthrough, you'll see your default API key included in the code snippets you can copy into your own application.
Additionally, you can find your default API key on the Algorithmia home page on the right-hand side.
You can find out more about creating and using API keys in our Developer Center using the link below.